It is known to use an optical fibre sensor in a control system, for example a control system for an industrial machine, an automatic door, or an electrically powered vehicle window. The optical fibre is placed and arranged so that it is deformed using the phenomenon of microbending when an abnormal condition arises, such as a machine operator treading on a pressure mat incorporating the fibre placed in an exclusion zone around the machine, or a person's body being hit by an edge of an automatic door incorporating the fibre or being trapped between the edge of a vehicle window and the window frame. A series of optical pulses can be transmitted along the fibre from a source such as a light emitting diode to a sensor such as a photodiode, and the amplitude of the output of the sensor is processed and monitored. When an abnormal condition arises and microbending occurs, the transmittance of the fibre is reduced. This reduction in transmittance is detected and the control system responds by rendering safe the apparatus which it controls, for example by powering off the machine, door or window, or by opening the door or window. In the known arrangement the optical and electrical system can be designed to be "fail-to-safe", that is to say any fault which arises in the system should result in the apparatus which it controls being placed in the safe state. Optical fibre sensors are particularly advantageous in such a fail-to-safe system. One such system is described in European published application EP 031474A, assigned to the present assignees.
Referring to FIG. 10, the general arrangement of one such system is schematically illustrated; it comprises a control unit 1 connected via an electrical cable 2 to a transducer unit 3. The transducer unit 3 includes an optical emitter circuit 12 powered from a signal generator 10 in the control unit 1 via the cable 2, and an optical receiver 20 (e.g. a photo diode or photo transistor) generating an electrical signal for transmission through the cable 2 to a detector and control circuitry 22, 25 in the control unit 1. Coupling the emitter 12 and the receiver 20 is an optical fibre sensor 14. A device 26 is controlled by the control circuitry 24, in dependence upon a sensed condition.
In some industrial applications, the cable 2 can be up to 50 meters long. In environments with high levels of ambient electromagnetic or radio frequency interference pollution (such as factories or other industrial environments, or automobiles) major problems arise in this known system. The interference has a negligable effect on the emitter circuit 12, which can be pulsed with currents of 1 amp or more to drive an LED, but the detector circuit 20 may have a working current of micro amps and is very much more susceptible to interference.
We have found that in such high levels of interference, the control unit 1 can therefore respond to interference spikes or pulses induced in the cable 2 as if they were pulses produced by the receiver 20 in response to optical pulses. This is a dangerous condition, in that even if the abnormal condition has reduced the transmittance of the fibre 14 to a level where no optical pulses are detected by the receiver 20, the control unit 1 can continue to respond to spurious pulses and hence fail to sense the abnormal condition.
One approach to the problem is to fail-to-safe by switching out the control circuit all together at a level where it cannot discriminate between the interference and the genuine signal. Whilst this is safe, it is undesirable since the circuit no longer operates, which in an industrial environment can mean down time on high volume production lines.
The electrical and optical pulses in the known system are conveniently substantially rectangular wave pulses of finite amplitude and period, and the interference appears as high amplitude spikes of very short period. It has been considered to use conventional passive or active filters in the electrical system to remove the interference, but such filters would also remove the high frequency harmonics making up the substantially rectangular wave pulses and cause distortion and damping of the proper pulses. Another problem with such an approach is that it is well known that improving the characteristic of the filter involves adding further components or orders. However, fail-to-safe systems must satisfy requirements as to the failure of components such that components cannot fail (either by short-or open-circuit) in a manner which leads to unsafe operation. Adding further components therefore reduces the intrinsic failure-safety of the apparatus, so that good filter performance and intrinsic reliability are opposing requirements.
It has also been considered to use phase locking of the electrical signal sensed by the receiver 20 with the originating electrical signal powering the emitter 12, that is to say comparing the two signals so that the proper pulses and the interference spikes can be distinguished. However, this method of verifying the sensed signal is not acceptable in a fail-to-safe system, because under certain fault conditions in the comparison circuitry the originating electrical signal could be passed to the control unit 1 so that the system would not be fail-to-safe.